The present invention relates to a process for producing a xcex2-butyrolactone homopolymer or copolymer having the characteristics of biodegradability and biological absorption and being useful as a functional polymeric material, as well as polymers obtained therefrom. In particular, the present invention relates to a process for producing a xcex2-butyrolactone homopolymer or copolymer having an extremely high molecular weight never achieved which comprises ring-opening polymerization of specifically pretreated xcex2-butyrolactone alone or ring-opening copolymerization thereof with other lactones in the presence of a specific catalyst, as well as polymers obtained thereby.
In recent years, it is reported that there are a large number of microorganisms producing and accumulating therein a polymer consisting of 3-hydroxybutyric acid units, derived from xcex2-butyrolactone, particularly (R)-3-hydroxybutyric acid units[see P. A. Holmes, Phys. Technol., Vol. 16, p. 32 (1985); xe2x80x9cBiodegradable Polymeric Materialxe2x80x9d, p. 26, written by Yoshiharu Doi and published by Kogyo Chosakai Publishing Co., Ltd. (1990)]. It is reported that the microbially produced polymer consisting of (R)-3-hydroxybutyric acid units and having a number average molecular weight as high as 100,000 to 1,000,000 is a thermoplastic resin [Microbial Polyesters, p. 2, written by Yoshiharu Doi, VCH Publishers (1990)].
The polymer consisting of 3-hydroxybutyric acid units is biodegradable, enzymatically hydrolyzable and biocompatible, and thus attracts attention as a functional material, and its production process using microorganisms or enzyme reaction has been attempted [see xe2x80x9cBiodegradable Polymeric Materialxe2x80x9d written by Yoshiharu Doi and published by Kogyo Chosakai Publishing Co., Ltd. (1990); JP-A-4-292619]. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.) However, the production process using microorganisms or enzyme reaction requires cumbersome procedures in e.g. preparing special microorganisms and separating the formed polymer from the microorganisms, and further the amount of the polymer produced is low, resulting in low productivity and high costs.
Besides the process using microorganisms, a wide variety of processes for producing poly(3-hydroxybutyric acid) by chemical ring-opening polymerization of racemic xcex2-butyrolactone have been proposed, and such prior art includes: (1) a method using a catalyst system consisting of triethyl alminium and water [R. A. Gross et al., Macromolecules, Vol. 21, pp. 2657-2668 (1988)], (2) a method using a catalyst system consisting of diethyl zinc and water [Y. Zhang et al., Macromolecules, Vol. 23, pp. 3206-3212 (1990); N. Tanahashi et al., Macromolecules, Vol. 24, pp. 5732-5733 (1991)], (3) a method using an aluminum-porphyrin complex as a catalyst [S. Asano et al., Macromolecules, Vol. 18, pp. 2057-2061 (1985)], (4) a method using a potassium compound or a solution thereof as a polymerization initiator [Z. Jedlinski et al., Macromolecules, Vol. 18, pp. 2679-2683 (1985)], (5) a method using a metal alkoxide with magnesium, tin or the like as a polymerization initiator [H. R. Kricheldorf et al., Macromolecules, Vol. 21, pp. 286-293 (1988)], and (6) a method using a 6-membered cyclic tin compound or a spiro cyclic tin compound as a polymerization initiator [H. R. Kricheldorf et al., Macromolecules, Vol. 28, pp. 6718-6725 (1995); H. R. Kricheldorf et al., Macromolecules, Vol. 29, pp. 8689-8695 (1996)].
However, the racemic xcex2-butyrolactone polymer i.e. racemic poly(3-hydroxybutyric acid) obtained in the conventional methods (1) to (6) described above is a low-molecular polymer with a weight average molecular weight of 39,000 or less, so it cannot be used to produce films or other products smoothly, and even if films etc. are obtained, these products are inferior in physical characteristics and poor in practical usability.
Under these circumstances, the present inventors have made studies to permit a xcex2-butyrolactone polymer having practically durable physical characteristics with a high molecular weight and being effectively usable for various molded articles and other purposes to be produced with good productivity by a chemical method without using any microorganism. As a result, the present inventors found that a xcex2-butyrolactone homopolymer [poly(3-hydroxybutyric acid)] having a considerably higher molecular weight (weight average molecular weight of about 100,000 to 860,000) and a copolymer of xcex2-butyrolactone with other lactones having a considerably higher molecular weight (weight average molecular weight of about 100,000 to 400,000) than those achieved in the conventional synthesis methods described above can be obtained by ring-opening polymerization of xcex2-butyrolactone alone, or of xcex2-butyrolactone with other lactones, in the presence of a specific catalyst, that is, one or more tin compounds selected from specific distannoxane derivatives having halogen atoms and/or isocyanate groups, carboxylic acid tin compounds and di-lower alkyl tin oxides, and the present inventors have previously filed patent applications therefor (JP-A-6-256482, JP-A-6-329768, and JP-A-8-53540).
On the basis of these applications, the present inventors have made further extensive examination. As a result, the inventors found that a xcex2-butyrolactone homopolymer or copolymer having a considerably higher molecular weight than those achieved in the present inventors"" methods described in JP-A-6-256482, JP-A-6-329768, and JP-A-8-53540 can be obtained with high yield and good productivity by polymerization of xcex2-butyrolactone alone or copolymerization thereof with other lactones by using as a catalyst at least member selected from tin compounds represented by the general formulae (1) and (2) below, following removal of acids and water contained in the xcex2-butyrolactone, and the present invention was thereby completed. Both the presently obtained xcex2-butyrolactone homopolymer and copolymer have a high weight average molecular weight of 500,000 or more, sometimes 1,000,000 or more, and in particular, the xcex2-butyrolactone homopolymer having a weight average molecular weight exceeding 860,000 can be easily obtained.
Accordingly, the present invention relates to a process for producing a xcex2-butyrolactone homopolymer or copolymer which comprises ring-opening polymerization of xcex2-butyrolactone alone, or xcex2-butyrolactone with other lactones, in which the ring-opening polymerization is conducted using xcex2-butyrolactone subjected previously to acid removal and dehydration treatment and in the presence of one or more compounds selected from the group consisting of tin compounds (referred to hereinafter as tin compound (1)) represented by the general formula (1): 
wherein R1 and R2 independently represent an alkyl group containing 1 to 12 carbon atoms which may have a branched chain, or a cycloalkyl group containing 5 to 7 carbon atoms, or a phenyl or naphthyl group which may have a substituent group, R3 represents an alkylene group containing 2 to 12 carbon atoms which may have a substituent group, a cyclopentane-1,2-ylene group or a cyclohexane-1,2-ylene group, and n is an integer of 1 to 10,
and tin compounds (referred to hereinafter as tin compound (2)) represented by the general formula (2): 
wherein R4 to R7 independently represent an alkyl group containing 1 to 12 carbon atoms which may have a branched chain, or a cycloalkyl group containing 5 to 7 carbon atoms, or a phenyl or naphthyl group which may have a substituent group.